Field of the Invention
The present invention relates to an imaging apparatus, an imaging system, and a method of driving an imaging system.
Description of the Related Art
There are described in Japanese Patent Application Laid-Open No. 2010-016416 an imaging apparatus that includes a plurality of pixels arranged in matrix and a column amplifier unit provided for each column of the plurality of pixels to output a pixel signal amplified by the column amplifier unit, and an imaging system that includes a signal processing unit for processing the output signal. The imaging system of Japanese Patent Application Laid-Open No. 2010-016416 is capable of amplifying, in the column amplifier unit, a signal output from a relevant pixel at a gain of q times, q being larger than 1, and subsequently multiplying, in the signal processing unit, the resultant image signal by a ratio smaller than 1.
When the quantity of incident light is small, the imaging system of Japanese Patent Application Laid-Open No. 2010-016416 can reduce the N component of the signal-to-noise (S/N) ratio by reducing input-referred noise (a value calculated by dividing noise that is generated downstream of the column amplifier unit by gain) through amplification at a gain of q times. When the quantity of incident light is large, the imaging system can prevent the output signal from saturating by reading at a gain p, which is smaller than q. The imaging system described in Japanese Patent Application Laid-Open No. 2010-016416 is thus capable of improving the S/N ratio and expanding the dynamic range.
A method of driving the imaging apparatus is also disclosed in Japanese Patent Application Laid-Open No. 2010-016416, which involves sequentially reading two signals that are varied from each other in gain. In a mode of the imaging apparatus described in Japanese Patent Application Laid-Open No. 2010-016416, output signals from the column amplifier unit are held sequentially in four holding capacitors (for example, FIG. 4 of Japanese Patent Application Laid-Open No. 2010-016416). The number of signals to be held is accordingly large, which gives rise to a problem of slow reading speed. Japanese Patent Application Laid-Open No. 2010-016416 also discloses, as another mode of the imaging apparatus, a configuration that includes two column amplifiers different from each other in gain to hold output signals in parallel in two pairs of holding capacitors. This configuration, however, requires two column amplifier units for one column of pixels, which gives rise to another problem in that the larger area taken up by the column amplifier units increases the chip area. Speeding up reading may therefore be difficult with this configuration in some cases.